Carrier-wave telegraphic transmission system



' March 4, 1930. w, FLUHARTY 1,749,739

Y CARRIEHWAVE TELEGBAPHIC TRANSMISSION SYSTEM Filed April 2. 1928 s sheets-sheet 1 I'W ajwaiuwz Max-h1 4, 193.0. w. G. FLUHARTY CARRIER WAVE 'IELEGRAPHIG TRANSMISSION SYSTEM 3 Sheets-Sheet 2 Ihwentor uttol neg Filed April 2. 1928 March 4, 1930. w, FLUHARTY 1,749,739

CARRIER WAVE TELEGRAPHIC TRANSMISSION SYSTEM Filed A a-11 2. 1926 s Sheets-Sheet 3 -s i A m I l -lin inventor a Bu ,Fiuharty Patented Mar. 4, 1 930- UNITED STATES PATENT OFFICE WILLIAM G. FLUHABTY, OF BOGKVILIBE CENTER, NEW YORK, ASSIGNOR TO THE WESTERN UNION TELEGRAIPH COMPANY, OF NEW YORK, N. Y., A CORPORATION .OF NEW YORK CARRIER-WAVE TELEGRAPHIO TRANSMISSION SYSTEM Application ma April 2,1928. Serial No. 266,654.

My invention relates to an improved keying arrangement for carrier wave telegraphic transmission systems.

HQI'BtOfOre in telgraphic transmission systems 'of the carrier wave type employing vacuum tube modulators or repeaters, the transmitting key l or rela controlling the modulator in accordance wlth the telegraphic signals was placed directly in the plate cir- 10 cuit of the tube, or directly in the grid or grid-battery-circuit of the tube. When so placed, the operation of the key or relay gives rise to disturbances commonlyknown in the art as.-key thumps. These disturbances or distortions transmitted by the carrier frequency, not only cause disturbances in the receiving set for which the signals are intended, but they also cause disturbances in I 4 other receiving sets tuned ,to'dilferent wave lengths. These disturbances are particularly objectionable in systems in which several carrier frequencies are used for the simultaneous transmission of several mes- I sages; where the key thump disturbances break throughthe several tuned receiving systems and cause false signals to be heard or recorded, or cause unnecessary distortion of the received signal. I

It is an object of my invention to devise a keyin system which will be free from the distur ances commonly known as key thumps.

A further object of my invention is to de vise a keying system in which the rate of rise of the envelope of each signal impulse may be controlled and given any desired slope.

My invention is illustrated in the accompanying drawing in which:

' Fig. 1 shows typical oscillographic records of a signal with the key thump and a signal without the key thump.

Fig. 2, is a circuit arrangement illustrating my invention as applied to a carrier wave transmitting system. Fig. 3, is an equivalent circuit arrangement of certain parts of the circuit of Fig. 2.

- Fig. 4, illustrates a modification of my invention.

Fig. 5, illustrates my invention as applied to a radio'telegraphic transmitting system.

by-pass condenser C is connected around Fig. 6, is a circuit diagram showing two transmitters supplied from common B and (3 current sources.

Referring to Fig. 2, O is an oscillator operating continuously at a predetermined frequency and supplying carrier current to transformenT connected to the input circuit of three-element vacuum tube 1. One terminal of the secondary S of transformer T is connected directly to the grid of the tube P, while the other terminal of the secondary is connected to one terminal of a condenser (L, and to one terminal of a resistance R The remaining terminal-of condenser C is connected to the grounded side of the filament of tube P. The remaining terminal of resistance R, is connected to ground through a resistance R and a grid biasing or 0 battery 18,. An auxiliary grid biasing or G battery B is arranged to produce a voltage drop across resistance R through a controlling resistance B A key K is connected in shunt to resistance R and is arranged to short-circuit this resistance in accordance with the telegraphic signals. The output circuit of tube P is provided with a transformer T the primary winding of which is connected in the plate circuit ofthe tube in series with the plate battery B A so the plate battery B c The battery B, is the usual grid biasing or C battery and is adjusted in potential to cause the tube P to operate substantially at the middle of the straight portion of the plate current characteristic curve. The battery B is so connected that the potential drop across resistance R tends to make the grid more negative. The magnitude of the voltage drop across resistance R may be adjusted to any desired value by the proper selection of the values of the two resistances R and R and of the value of the potential of battery B The operation of the system shown in Fig. 2 is as follows I, 9.,

With the key K open, the combined potentials of C battery B and the negative potential drop across resistance R is of such value that substantially no plate current flows through tube P. Thus, with key K 100 open, there is no transmission of carrier waves from oscillator through the modulator tube P. Upon closing key K, the voltage across resistance R is short-circuited. Prior to the closing of key K condenser G was charged to a potential equal to the combined voltages of C battery B and the voltage drop across resistance R Upon closing key K, the condenser C discharges through the circuit including resistance R and battery B that is, the condenser discharges through this circuit until its terminal voltage becomes equal to the voltage across battery 13,. Vhen steady conditions are attained within the circuit, the biasing potential upon the grid is equal to that supplied by the battery B and carrier waves are repeated and amplified from oscillator through the modulator or relay tube P to the output circuit. Due to the fact that the potential impressed upon the grid of tube P is equal to the voltage across the condenser G at any instant, and due to the fact that the condenser does not discharge instantaneously, the transmission of the carrier currents through the tube is not initiated abruptly but in a smooth manner. The grad ual rise or shaping of the envelope which comprises the signal, may be controlled by controlling the rate of discharge of the condenser C when the key K is closed.

Upon opening the key after the transmission ofa signal character, an additional po tential is thrown in series with that of the battery B This potential is the drop across resistance R as mentioned above, and is of such magnitude and polarity that the total biasing potential on the grid of tube P 1s made so highly negative that transmission of carrier current through the tube ceases. However, the carrier current is not abruptly,

stopped due to the fact that condenser C does not immediately assume the combined potential of battery B and the drop across resistance R but a short time is required for the condenser to become fully charged and to assume its maximum voltage. Thus, manipulation of key K starts and stops the transmission of carrier current through the tube without breaking or interrupting any of the main circuits. Also, by virtue of applying the auxiliary biasing battery at the low potential side of the tube circuit, the values or characteristics of the tube circuits are not disturbed, and there is no distortion of the transmitted signals other than those caused by the inherent characteristics of the tube used and its adjacent circuit.

The time rate of change of the potential across condenser C may be controlled by properly choosing the values of the condenser and of resistance R and other constants of the associated circuit. The rate of change of potential across condenser C may be predetermined 'by the application of well known and demonstrated formulas. As the poten- Fig. 3 is the equivalent circuit of that part of the circuit in Fig. 2 which has to do with the transient voltages. When considering the operation of the transient voltages, we are concerned only with the change of voltage across the condenser C consequently the remainder of the circuit of Fig. 2 may be ignored.

Referring to Fig. 3: some time after the key K has remained closed there will be no potential across the condenser C as the charge will have equalized through the resistance R and the key, also the potential across R is short-circuited. At the instant the key is opened the potential across R is released and starts to charge the condenser G at a rate expressed in the following well known formula found in standard hand books on the subject.

V=voltage across the conductor at any time after the keyis opened.

E =1mpressed voltage, or voltage across R t=t1me in seconds.

V=voltage across condenser before opening key. y

As there was no yoltage across the condenser C just before the key was opened the above formula reduces to 0 ".5 microfarads or m farads.

Solving for R in the above equation we have 1 Oge E )a Substituting the correct values in the foregoing equation we have The proper resistance to use for R is then found to be 7070 ohms. At the end of .01 of a secondafter the key is opened 10 volts biasing grid potential has been added to the orig- ,inal biasing 0 battery shown as B in Fig.

can be seen to gradually die out to zero as the potential builds up across the condenser C The voltage discharge transient of the condenser C starts at the instant the key is closed and may be calculated from the following formula in which the previous values may be substituted.

V =voltage of condenser G at a time 15 2 1 volts.

' t=time in seconds requlred for voltage to drop to 2.

- V=voltage charge on condenser at instant of closing key=12 volts.

R 7070 ohms.

0=.5 microfarads From the above formula t=log(%)(RC) substituting t= 1.221. 7070 x .5 X 10 .00434 second, required for voltage on condenser C to drop to 2 volts.

It will be noticed by referring to Fig. 1 that when the voltage across the condenser 0 is least the amplitude of the oscillations is maximum and when the voltage across the condenser C is maximum the oscillations are minimum. If the polarity of the battery B is reversed the efiect on amplitude of the oscillations is also reversed. In the calculations the potential of the customary grid battery was neglected. This is per.- missib e I alteredby opening or closing the key. The

because the potential of B is not when they are in opposite directions.

In Fig. 1, the curve (a) shows an oscillographic record of a dot signal when resistance R is reduced to zero, and curve (1')) shows the same signal when R is 1000 ohms.

The key thum disturbances are clearly shown in curve a), while they are not to be found in curve (6). The wave in curve (2')) rises gradually and" smoothly to maximum value.

'An inductance in series with a resistance may be substituted for R in Fig. 2. In that case the transient will be changed according to the values used; that is, the rate of charge and discharge of the condenser C will be changed according to the values of the units selected, but the action on the transient voltage across the condenser C will function the same as in the case previously described. Fig. 4 illustrates this arrangement. Actual; values may be calculated from well known formulas.

The main feature of this keying scheme is that single frequencies or a combination of multiple frequencies may be transmitted through the tube P and the rate of rise and fall of the envelope of these frequencies reg-- ulated without causing distortion to these oscillations by the regulating means; or disturbance in the oscillating or tuned circuits by the addition of these keying means.

When the frequency of the oscillations is sufliciently removed from the keying trancapacity and resistance may be substituted in the place of R of Fig. 2 without departihg from the main features of this scheme. Also, different combinations of inductance, capacity and resistance may be substituted in place of condenser 0 without departing from this scheme.

'My keying arrangement may be directly applied to an oscillating tube system to start and stop theoscillations of the tube generator and control the transient or envelope of the transmitted or generated wave according to predetermined constants. This keying scheme is especially applicable to radio transmission systems to remove the so-called key thump from the transmitted signals. When applied to transmitting systems it accomplishes a sharper transmitted wave of the carrier frequency and a more pleasing and understandable signal. Also, because of thetion.

In installations where two or a number of simultaneous carrier frequencies are used, such as in multiple frequency carrier telegraphy, it would be more convenient to use one common battery or generator to furnish the C biasing and keying battery for .the several transmitters. Fig. 6 shows two'transmitters being supplied from common battery. More transmitters may be supplied by carrying out. the same scheme of connections on the additional transmitters as shown in the figures I claim:

1. A signal transmission system comprising an electric discharge relay having a cathode, an anode and a control electrode, a source of space current for said relay, an input circuit, a source of carrier waves to be repeated connected to the input circuit, a source of biasing potential for normally biasing the control electrode at such potential that the carrier waves are not effectively repeated by the relay, means for abruptly changing the potential of the biasing source in accordance with telegraphic signals to render said relay operative, and means for gradually changing the potential of the control electrode in response to the abruptchanges of the biasing source voltage.

2. A si nal transmission system comprising an el ectric discharge relay having a cathode, an anode and a control electrode, a source of space current for said relay, an input circuit, a source of carrier waves to be repeated connected to the input circuit, a

source of biasing potential for normally biasing the control electrode at such potential that the carrier waves are not effectively repeated by the relay, said source of biasing potential including an energy storing device and a source of electric energy for charging the same, means for abruptly changing the potential of the source of electric energy, and means for controlling the rate oi charge and discharge of said storage device to prevent abrupt changes in the potentialof the control electrode.

3.1A signal transmission system comprising a thermionic relay having a cathode, an anode and a control electrode, an output circuit for the relay including a source of space current, an input circuit connected between the cathode and the control electrode and including a condenser, a source of biasing potential connected in shunt to said condenser, means in said shunt and associated with said source of biasing potential for changing the voltage of said source in accordance with telegraphic signals, and a resistance connected in said shunt path in serieswith said source.

4. A signal transmission system comprising a thermionic relay having a cathode, an anode and a control electrode, an output circuit for the relay including a source of space current, an input circuit connected between the cathode and the control electrode and including a condenser, a source of biasing potential connected in shunt to said condenser, means associated with said source of biasing potential for changing the voltage of said source'in accordance withtelegraphic signals, and a resistance and a choke coil connected in said shunt path in series with the source of biasing potential.

5. A signal transmission system comprising an electric discharge relay having a cathode, an anode and a control electrode, an output circuit for the relay including a source of space current,-an input circuit connected between the cathode and the control electrode including a condenser, a path in shunt to said condenser including a biasing battery and a resistance connected in series, means for producing a voltage drop across said resistance including a second battery and a second resistance connected in series across said first resistance, and means for short-circuiting said first resistance in accordance with telegraphic signals.

6. A si nal transmission system comprising an electric discharge relay having a cathode, an anode and a control electrode,

an output circuit for relay including a source of space current, an input circuit connected between the cathode and the control electrode including a condenser, a path in shunt to said condenser including a biasing battery and a resistance connected in series, means for producing a voltage drop across a portion of said including a condenser, a path connected in shunt to said condenser including a biasing battery and two resistances connected in series, means for producing a voltage drop across one of said resistances including a battery and a third resistance connected in series across the terminals of said last mentioned resistance, and means for short-circuiting the energized resistance in accordance with telegraphic signals. 8. A telegraphic signaling system comprising a source of carrier waves, a modulator for said waves including a control circuit, a

source of signal impulses connected to said control circuit, a condenser in shunt to said control circuit and a high resistance in series with said circuit between the condenser and the source of signal impulses.

9. A telegraphic signaling system comprising a source of carrier waves, a modulator for said waves including a control circuit, a source of signal impulses connected to said control circuit, a condenser in shunt to said circuit, and a high resistance and a choke coil in series with said circuit between the condenser and the source of signal impulses.

10. A telegraphic'signaling system comprising a source of carrier waves, a modulator for said waves having a control circuit, a source of signal impulses, a translating device, means for discharging the signal impulses through said translatin device, means 7 for controlling the rateof dlscharge of the signals through the translating device, and circuit connection for supplying the control circuit of the modulator with current derived from across the translating device.

11. The method of controlling a modulator in accordance with telegraphic signals which consists in, producing sharp voltage pulses in accordance with the signals, discharging the pulses through a translating device, controlling the rate of discharge to obtain a gradually increasing potential drop across the translating device, and supplying the control circuit of the modulator with current derived from across the translating device.

12. The method of controlling a modulator 1 in accordance with telegraphic "signals which consists in, producing sharp voltage pulses in accordance with the signals, charging a condenser with each pulse, controllingthe rate of charge to obtain a gradually increasing voltage across said condenser for each pulse, and supplying the control cir cuit of the modulator with current derived from across the condenser.

In testimony WhereofI aflix my signature.

WILLIAM G. FLUHARTY 

